In general, flexible printed wiring boards (flexible printed circuit boards) have a basic structure in which a copper foil or the like is bonded with an adhesive to one surface or both surfaces of an insulating film functioning as a base, the insulating film being composed of a heat-resistant film such as a polyimide film. As such an adhesive, hitherto, an adhesive obtained by mixing a flame retardant with a blend resin of a thermosetting resin such as an epoxy resin and a thermoplastic resin such as an acrylic resin, a polyamide, or a polyester resin has been used.
As the flame retardant, halogen flame retardants have been hitherto used because high flame retardancy corresponding to the VTM-0 class or the V-0 class in the Underwriters Laboratories Inc. (UL)-94 standard is required. Recently, however, in view of the problems of environmental pollution, phosphorus flame retardants such as phosphoric acid esters, phosphoric acid ester amides, melamine polyphosphate, ammonium polyphosphate, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and derivatives thereof, and phosphazene compounds have been used instead of the halogen flame retardants.
However, in order to satisfy high flame retardancy corresponding to the VTM-0 class or the V-0 class in the UL-94 standard by using only such phosphorus flame retardants, it is necessary to mix a large amount of a phosphorus flame retardant as compared with the case where a halogen flame retardant is used. This results in a problem that, with an increase in the amount of phosphorus flame retardant mixed, the adhesiveness decreases.
In order to solve this problem, recently, it has been proposed that the amount of phosphorus flame retardant mixed be reduced by using a resin that utilizes a flame retardant effect of phosphorus.
For example, PTL 1 (Japanese Unexamined Patent Application Publication No. 2003-176470) has proposed that the phosphorus content in a composition is controlled to be 2% by weight or more by using a phosphorus-containing epoxy resin and further using a phosphorus-containing phenoxy resin as a portion of a thermoplastic resin.
In addition, PTL 2 (Japanese Unexamined Patent Application Publication No. 2005-53989) discloses a flame-retardant adhesive resin composition containing a blend resin of a halogen-free epoxy resin and a phosphorus-containing polyester resin, a phosphazene compound functioning as a flame retardant, and inorganic filler such as magnesium hydroxide or aluminum hydroxide. It is described that flame retardancy and solder heat resistance can be satisfied by controlling a content ratio of phosphorus element to the resin component to be in the range of 1.8% to 5% by weight even without using a phosphoric acid ester. Also, PTL 2 describes that a phosphorus-containing polyester resin having a weight-average molecular weight of 10,000 to 50,000 can be used, and discloses Examples in which VYLON 537 (weight-average molecular weight: 140,000) and VYLON 237 (weight-average molecular weight: 30,000) manufactured by Toyobo Co., Ltd are used.
Furthermore, PTL 3 (Japanese Unexamined Patent Application Publication No. 2007-254659) has proposed an adhesive resin composition for a flexible printed wiring board, the adhesive resin composition containing a blend resin of a phosphorus-containing epoxy resin and a thermoplastic resin having a solubility parameter of 8 to 16 and, as a flame retardant, a phosphorus-containing polyester resin that has a weight-average molecular weight of 2,000 to 20,000 and that is soluble in an organic solvent. PTL 3 discloses that a sufficient adhesive force can be ensured by using the phosphorus-containing polyester resin having a weight-average molecular weight of 2,000 to 20,000 without impairing solder heat resistance and flame retardancy, as compared with the case where a phosphoric acid ester flame retardant or a phosphoric acid ester amide flame retardant is used. It is described that, on the other hand, a phosphorus-containing polyester resin having a weight-average molecular weight exceeding 20,000 has poor solvent solubility.